I'm a newbie looking to build my first decent HT system, and I found 2 pairs of lightly used Definitive Tech ProCinema 1000 and 800 monitors as well as a 2000 center channel at a good price.

What subs would work well for this setup in a 12x14 home theater area in a larger 12x28 space?

The set would be used for 50% Bluray/DVD/Dish HD content, 30% PS3 gaming, and 20% music. I want something with some musicality that is strong but won't over-power the rest of the system. That said, I do own my house and neighbors aren't an issue.

Size and appearance are not overly of concern, but I don't want to spend much more than $500, and would be happy to spend less.

SVS, Hsu, Elemental Design, Epik, etc. Take your pick. You need not to worry about matching a sub to your speakers, just worry about a sub that can fill that space. You also need the height of the room because you need to know the volume of air that you will need to pressurize.